High-energy “eye safe” laser illumination is needed for various ranging and illumination applications most specifically with respect LIDAR and gated viewing systems. Of specific interest is single pulse identification at ranges exceeding 25 km using a 1.5 micron source packaged for airborne operation. The desired combination of high pulse energy, low beam divergence within the system packaging and operating constraints has proved challenging as reported by Larry R. Marshall, Alex Kax and Orhan Aytur, in an article entitled “Multimode Pumping of Optical Parametric Oscillators”, IEEE Journal of Quantum Electronics 32 177-182 (1996). It will be appreciated that airborne systems require and in fact mandate compactness, low weight and reliability.
In the past laser target designators have used Nd:YAG pumped KTP crystals utilized in an optical parametric oscillator which produces 60 mJ of energy when pumping the optical parametric oscillator with a 1 micron pumping source.
The task for military and other purposes requires a doubling of the output energy so as to provide a long-range eye safe laser for use in long range laser ranging and laser illumination applications. In order to provide the required energy on target, between 100 mJ and 200 mJ must be produced from the laser transmitter with a beam quality of less than 25 mm-milliradian. For this application, such performance must be obtained using a designator class laser, 300 mJ, 20 Hz, in a compact configuration, with a transmitting aperture less than 20 mm.
One of the first attempts to increase power for such eye safe lasers included simply adding additional KTP crystals in the oscillator cavity, with the suggestion of using three such crystals to boost the 60 mJ prior output power to a 150 mJ level.
However, this approach while generating the required output power, resulting in a beam spread of 80 milliradians, clearly an order of magnitude greater than that which is desirable. It is noted that the cross section of such a beam at 25 kilometers puts very little of the projected energy on target. Moreover, for target illumination purposes requiring high resolution, such beam widths are not readily unusable.
In an effort to reduce the beam divergence, it was suggested to pump an optical parametric oscillator followed by an optical parametric amplifier by dividing the pumping pulse into two pulses. The first pulse was to be redirected through the optical parametric amplifier, whereas the second pulse is directed through a pulse timing delay unit so as to appropriately pump the follow on optical parametric amplifier.
However, providing an optical delay takes up a significant amount of space and requires an increased parts count which can in some cases deleteriously affect the efficiency of the system due to alignment problems.
Also synchronizing the pulse time delay is non-trivial problem so that the pumping pulse from the pumping laser arrives at the optical parametric amplifier so that it is not pumped too soon. It would therefore be desirable to have a single beam line system which avoids alignment problems while at the same time eliminating beam redirecting optics and yet still have an intense output beam with a less than 1.2 milliradian beam divergence.
As will be appreciated, the above proposed optical parametric oscillator, optical parametric amplifier combination was initially ruled out due to the optical complexity and space needed to split and synchronize the pump beam.